Electric coupling circuits



Jan. 16, 1934. H. A. WHEELER 3,

I ELECTRIC COUPLING CIRCUITS Filed Sept. 10, 1950 INVENTOTR 194/7010 A. w/ ifzi/ QMAI$MI hum; M

ATTORN EYS Patented .lan. 16, 1934 7 1,943,495 npnc'rnre com-LING oiacorrs Harold A. Wheeler, Jackson Heights, N. Y., as-

signor to Hazeltine Co oi Delaware rporation, a corporation Application September 10, 1930 Serial No. 480,948

This invention relates to high frequency elec-' tric coupling circuits tunable over a range in frequency adapted more especially for interconnecting successive elements of a high frequency thermionic amplifier or radio system.

The coupling circuit of the present invention is characterized in that the voltage amplification produced thereby is caused to vary with the frequency of tuning in a manner which is different from that of the simple two-winding transformer.

With an elementary type of high frequency coupling circuit of which a two-winding transformer having a tunable secondary is a typical 15' example, the voltage amplification obtained increases with the tuned frequency throughout the frequency range of tuning, in a manner well understood and with consequent detrimental effect upon the uniformity of response, selectivity and stability in. the system.

In the mentioned elementary type of coupling circuit, there exists between the primary and tunable secondary circuits a coemcient of coupling which is substantially constant throughout the frequency range, a condition which-is partially responsible for the mentioned increase in amplification with frequency. The present invention avoids the above noted objections to the simple type of coupling circuit by the proper inclusion of a capacitive reactance therein adapted to provide an effective coupling between input and tunable output circuits which automatically decreases with increase in the tuned frequency, and thereby causes the amplification to vary in the reverse manner with frequency, that is, to be greater at lower frequencies, or to be nearly independent of frequency adjustment.

Coupling circuits broadly operative in accordance with the principles disclosed herein, but differing from the present invention as regards both the circuit connections utilized and the specific modes of operation thereof, are disclosed in a copending application of C. E. Trube, Serial No. 120,045, filed July 2, 1926.

The coupling circuit of the present invention comprises the following essential elements namely; a fixed capacity, a fixed inductance and the frequency of tuning is increased, an increasing proportion of an impressed signaling wave will be shunted through the fixed capacity leaving a decreasing proportion which flows through the inductance and tuning condenser. This arrangement provides uniformly high and sensitivity.

Referring now to the drawing:

Fig. 1 shows in diagrammatic form the elementary coupling circuit of the present inven: tion;

Fig. 2 shows the coupling circuit of Fig. 1 with an antenna circuit connected to the input terminals thereof; while,

Fig. 3'discloses the coupling circuit of Fig. 1

selectivity as employed for interconnecting a pair of thermionic tubes in cascade relation.

Like elements are similarly designated in the several figures.

f Referring to Fig. 1 the coupling circuit comprises the fixedcapacity C2, fixed inductance L and adjustable tuning condenser 0 connected in a closed series circuit. The fixed capacity Cz is bridged between the input terminal I, while the tuning condenser C is. bridged between output terminals 0.

In Fig. 2 an antenna 1 and ground 2 are connected to the upper and lower input terminals E respectively. The output terminals 0 may of course be connected to the input section of a thermionic tube, or coupled to a second tunable circuit. In the circuit of Fig. 2 the capacity C2 includes the total efiective antenna-to-ground capacity of the antenna circuit in addition to any supplemental condenser connected-between the input terminals.

In the modification of Fig. 3 the upper input terminal I of the coupling circuit is connected through a blocking condenser C3, of relatively large capacity, to the anode of a thermionic element V1, the cathode of which is connected directly to the lower input terminal I; while the output terminals 0 are connected to grid and cathode respectively of a second thermionic element V2. The coupling circuit therefore connects tubes V1 and V2 in cascade. Space current for tube V1 is supplied through a choke coil 3 connected directly between the anode and the positive terminal of a source of space current supply, the negative terminal of which is connected to the cathode as indicated by +3 and -'.B, respectively. The blocking condenser C3 serves merely to prevent the anode potential of tube V1 from affecting the grid circuit of tube Vi, and has n9 appreciable ell'ect upon the operation of the interstage coupling circuit.

The total capacity of Cz-is usually larger, some times much larger, than the maximum value of C.

Natural leakage may be sufficient to maintain the grid of tube V: at a suitable potential. Otherwise, a grid leak R of high resistance should be connected across C2 or C as shown in Fig. 3.

It will be noted that in the coupling circuit described, the rotor or other moving element of the variable condenser C may begrounded directly as indicated in Figs. 2 and 3. This is an advantage in that it eliminates certain accidental and undesired couplings, and also allows all condenser rotors of a multistage receiver to be mounted on a single grounded shaft.

What is claimed is:

1. A-high frequency electric coupling circuit comprising an input terminal, an output terminal and a common terminal, a fixed capacity connected directly between said input and common terminals, a variable condenser of smaller capacity connected between said output and common terminals, and an inductance connected between said input and output terminals, whereby there isformed a closed circuit tunable over a frequency range, the elements of said closed circuit being proportioned to provide uniformly high selectivity and sensitivity throughout said frequency range.

2. A high frequency thermionic amplifier comprising a pair of thermionic tubes each having anode, cathode and grid, and an electric coupling circuit connecting said tubes in cascade, said circuit comprising a ground terminal joining said cathodes in common, an input terminal electrically connected to the anode of onetube, an output terminal connected to the grid of the other tube, a fixed capacity connected directly between said input and ground terminals, a variable tuning condenser connected between said ground and output terminals, and a fixed inductance connecting said input and output terminals,

whereby there is formed a closed series circuit tunable over a range in frequency, the elements of said coupling circuit being proportioned to provide uniformly high selectivity and stability throughout said frequency range.

3. In a high frequency signal receiving system the combination of an electric coupling circuit having an input and a common terminal connected to an antenna circuit, an output and said common terminal connected to the input of a vacuum tube system, a fixed inductance connected between said input and output terminals, a variable condenser for tuning said circuit throughout a range in frequency connected be tween said output and common terminals, and a fixed condenser connected directly between said input and common terminals, said fixed condenser being so selected in relation to the capacity of said antenna circuit and to said fixed inductance as to provide uniformly high selectivity and sensitivity of energy transfer throughout said tunable frequency range from said antenna to said vacuum tube system.

HAROLD A. WHEELER. 

